1. Field of the Invention
This invention relates to a muffler for a small air-cooled two-stroke gasoline internal combustion engine which is suited for use in a portable small working machine such as a bush cutter or a chain saw. More particularly, the invention relates to an air-supplying device for a muffler in which an oxidation catalyst is accommodated in order to comply with exhaust gas regulations.
2. The Prior Art
In view of the recent increasing concerns over environmental issues, the reduction in the amounts of HC, CO, NOx, etc. in the exhaust gas discharged from an engine is now strongly required even for a small air-cooled two-stroke gasoline internal engine to be used in a portable small working machine such as a bush cutter or a chain saw. For example, according to the exhaust gas control bill in California known as CARB 1999, it is stipulated that CO should be reduced to 130 g/bhp-h or less, the total HC (THC) to 50 g/bhp-h or less, and NOx to 4 g/bhp-h or less, starting from 1999.
With a view to cope with such an exhaust gas regulation, there have been proposed various measures, such as improving the combustion chamber of internal combustion engines, modifying the shape of the suction/exhaust ports or of the scavenging port of cylinders of internal combustion engines, and providing an exhaust gas system with an exhaust gas-purifying means such as an oxidation catalyst. However, all of these attempts have failed to obtain a satisfactory result to date.
The present assignee of the present invention has already proposed, as a countermeasure to cope with the aforementioned exhaust gas regulation, a system wherein the exhaust gas discharged from the exhaust port of an engine is directly introduced into a muffler disposed in the engine body, and wherein the muffler is provided with an oxidation catalyst in a specific manner for purifying the exhaust gas. Specifically, the measures set forth in the Japanese Patent Application H7-272959 (U.S. patent application Ser. No. 08/733,299), now U.S. Pat. No. 5,722,237, are constituted such that an oxidation catalyst assembly is formed at first by superimposing a web-like oxidation catalyst on a spark arresting metal mesh having a peripheral folding portion to obtain a laminate. The peripheral folding portion of the metal mesh is then turned around the entire or most of the periphery of the laminate, thereby providing a U-shaped or V-shaped oxidation catalyst assembly, which is then disposed within the muffler (Prior Art No. 1). The measures set forth in the Japanese Patent Application H7-343092 are constituted such that an oxidation catalyst comprising an air-permeable metallic foamed body is disposed in an exhaust gas inlet portion of the muffler (Prior Art No. 2).
According to the muffler incorporated therein with an oxidation catalyst as proposed by the Prior Art No. 1 and 2, the structure of the muffler itself, the shape of the oxidation catalyst, and the layout of the catalyst are not yet optimized to sufficiently enhance the exhaust gas purifying property of the catalyst. Further, measures for improving the mounting property, assembling property and design freedom of the muffler in relation to the engine are not taken into account. Moreover, special consideration is not given to the problem resulting from the increase in temperature of the exhaust gas due to the reaction thereof with the oxidation catalyst (oxidative combustion).
Under the circumstances, the present applicant has proposed an improved muffler for a small air-cooled two-stroke gasoline internal combustion engine as set forth in the Japanese Patent Application H8-84260 and the Japanese Patent Application H8-84324 (U.S. patent application Ser. No. 08/773,978, now U.S. Pat. No. 5,738,184, which take into account the aforementioned problems. Namely, this improved muffler is provided with an expansion chamber to which an exhaust gas ejected from the exhaust port of an engine is introduced. The expansion chamber is perpendicularly separated by means of a partition plate into a first expansion chamber and a second expansion chamber. This partition plate is provided with an oxidation catalyst, which is composed of a metallic foamed body having gas-permeability. Accordingly, an exhaust gas is initially introduced into the first expansion chamber and then passed via the oxidation catalyst to the second expansion chamber. In the case of the Japanese Patent Application H8-84324 in particular, an outside air intake means is mounted near the exhaust gas inlet portion of the first expansion chamber. This outer air intake means is designed to introduce outside air into the expansion chamber by taking advantage of the jet flow of the exhaust gas into the expansion chamber.
Although this modified muffler is designed to introduce the outside air by taking advantage of the jet flow of the exhaust gas, it is difficult to introduce a sufficient quantity of air to sufficiently reduce the CO component even with the force of the jet flow of the exhaust gas. Namely, if the CO component in the exhaust gas is to be minimized, the CO component is required to be reacted with the O.sub.2 in the air so as to generate CO.sub.2. However, if the quantity of air introduced into the muffler by taking advantage of the jet flow of the exhaust gas is not sufficient, the quantity of O.sub.2 available for the reaction will also be insufficient, so that the reaction of CO to produce CO.sub.2 is limited, thus making it difficult to sufficiently reduce the CO component.
On the other hand, in the case of four-stroke engines employed in a motor car or a motor bicycle, there has been proposed to employ an air pump which is actuated by means of power for feeding outside air to the muffler. However, since the air pump itself is relatively expensive and heavy, requiring a special driving device, it is not suited for use with a muffler for a small air-cooled two-stroke gasoline internal combustion engine which is suited for use in portable small working machines.